This invention relates to a programmable timing circuit for a cathode ray tube.
As is well known, a cathode ray tube requires various timing signals for its correct operation. Typically signals are required at start/stop horizontal synchronization, start/stop blanking, end of raster scan line, end of frame, cursor, indicator row, etc. Early cathode ray tubes had dedicated timing circuits but in more recent years programmable timing circuits have been provided. These can be adapted to a particular CRT display by loading various parameters into the circuit.
Typical of such a programmable timer is that sold by Motorola Inc. as the Motorola 6845 circuit. In prior art timers, each programmable timer normally requires its own register and comparator which compares the contents of the register continuously with a counter. Whenever a comparison is detected, the appropriate timing signal is derived.
Patent Specification GB-A No. 2,075,791 describes a programmable timing signal generator which includes a small random access memory in which each word stored corresponds to a timing state and each output bit provides a sync video related signal. This is not a generator designed to give control outputs at particular points on the screen, having no character, row or line counters. It produces complete pulse sequences in short high resolution bursts separated by long time intervals.